Dhall-lang: Add Text/split primitive

[jvanbruegge]

It would be cool to have some way to do search + replace on Text values and the simplest way to do it I can think of is a Text/split primitive.

let split : Text -> Text -> List Text = \sep -> \input -> Text/split sep input

with this, replacing a character (or a longer sequence) can be defined in dhall (could be added to the prelude):

let replace = \(old : Text) -> \(new : Text) -> \(input : Text) ->
    prelude.`Text`.concatSep new (Text/split old input)

I don't have time right now to implement this myself, so I am opening this issue

[Gabriel439]

I've been holding off as long as possible on standardizing support for parsing and introspection on Text values. The main reason is to catch more input failures at type-checking time rather than at run time (i.e. via Optional values). Also, I prefer requiring that the input is a more structured representation than a string in order to ensure that invalid inputs are unrepresentable

However, I do maintain a wiki page explaining how you can add your own built-ins if necessary here: https://github.com/dhall-lang/dhall-lang/wiki/Built-in-types,-functions,-and-operators

[jvanbruegge]

Yeah, I think the current design works well and I agree that this should be held off